1. Field of the Invention
The present invention relates to a thermosetting organosiloxane composition which cures by means of the hydrosilylation reaction. More particularly, the present invention relates to a hydrosilylation-curing thermosetting organosiloxane composition which has an excellent storage stability at temperatures close to room temperature but which also exhibits excellent curing properties at elevated temperatures.
2. Description of the Prior Art
Because they cure rapidly even in deep sections without generating reaction by-products, hydrosilylation reaction-curing organosiloxane compositions are employed throughout a broad range of fields, for example, as adhesives, as potting and coating materials for electric and electronic components, and as release coatings for papers and films.
However, such organosiloxane compositions suffer from an exceedingly poor storage stability, and they cannot be stored enclosed in a single container. As a consequence, they must generally be stored with their ingredients divided up into at least two containers, one of which contains the organohydrogenpolysiloxane and the other the hydrosilylation reaction catalyst. In order to avoid this problematic attribute, it has been proposed that the catalytic activity of the hydrosilylation-reaction catalyst, and particularly of platinum-containing catalysts, be restrained or suppressed. One such approach comprises the use of an additive which has the capacity to restrain or check the catalytic activity of platinum-type catalysts, and such additives are exemplified by benzotriazoles, acetylenic compounds, hydroperoxy compounds, etc. Unfortunately, when a long-term storage stability is sought using such methods, the curing properties are compromised and, among other things, the time required for curing at elevated temperatures is lengthened. While an curable organosiloxane composition with an excellent storage stability is in fact obtained, it will not be a rapidly curing organosiloxane composition.
The prior art has attempted to improve the storage stability of platinum-containing hydrosilylation catalysts. One method for accomplishing this is disclosed in Japanese Patent Application Laid Open [Kokai] Number 49-134786 [134,786/74], which teaches the preparation of a platinum-type catalyst in powder form. Silicone resin and a platinum compound catalyst adsorbed on a finely divided material such as quartz powder, are mixed and kneaded on a hot roll, and the resulting mixture is pulverized or ground.
U.S. Pat. No. 4,481,341, which issued to Schlak et al. on Nov. 6, 1984 describes thermosetting organosiloxane compositions comprising a polyorganosiloxane containing at least two ethylenically unsaturated hydrocarbon radicals per molecule, a polyorganohydrogensiloxane containing at least two silicon bonded hydrogen atoms per molecule and a platinum-containing catalyst that is dispersed in a finely divided, solid material, such as a silicone resin, at a concentration of from 0.001 to 5 percent by weight of platinum metal.
The finely divided material in which the catalyst is dispersed is virtually insoluble in either the aforementioned polyorganosiloxane or polyorganohydrogensiloxane and melts or softens at a temperature between 70 and 250 degrees C. The alleged advantage of these compositions disclosed by Schlak et al. is that the catalyst remains isolated from the other ingredients of the curable composition until the composition is heated sufficiently to melt the material in which the catalyst is dispersed. Because the organosilicon compounds present in the composition will not cure in the absence of the catalyst, the composition can allegedly be stored for long periods of time without undergoing curing or even an increase in viscosity.
A disadvantage of Schlak et al. catalyst resides in the method used to prepare the catalyst composition. A solid block or sheet of resin containing the platinum composition dispersed throughout is ground to a fine powder. The random nature of the grinding operation makes it possible that some of the particles will contain platinum catalyst on their surface. Even trace amount of platinum have been shown to cause premature curing of the type of organosiloxane composition exemplified in this patent.
U.S. Pat. No. 4,784,879, which issued to Lee et al. on Nov. 15, 1988 describes the preparation of an encapsulated form of a platinum-containing hydrosilylation catalyst. The platinum-containing catalyst is encapsulated within one or two layers of thermoplastic organic polymers. The catalyst is prepared by polymerization or precipitation of the encapsulating polymer in the presence of the catalyst. The resultant microcapsules are then washed with a liquid that is a solvent for the catalyst but not for the encapsulating polymer. The exemplified compositions require one washing with methanol and one with cyclic dimethylsiloxane oligomers to ensure their storage stability when incorporated into a curable organosiloxane composition.